Plant Kingdom: Class 11 Biology · Chapter 3

Scrape the green slime off a pond stone, run a finger over the velvet that coats a compound wall after the monsoon, turn over a fern leaf on a hill trek, pick up a fallen pine cone in Shimla, then bite into a mango on the way home. Every one of these organisms belongs to Kingdom Plantae. This chapter walks through the kingdom group by group, from algae to flowering plants, and keeps asking two questions: what is new in each group, and how do botanists decide where a plant belongs?

What counts as a plant today

In Chapter 2 you met R.H. Whittaker's five kingdoms (1969): Monera, Protista, Fungi, Animalia and Plantae. The older two-kingdom schemes had a far bigger plant kingdom. Almost anything that had a cell wall and did not move about was filed under plants, so bacteria, fungi and the photosynthetic 'blue-green algae' were all called plants Fig. 3.1.

Plantae is now much narrower. Fungi have a kingdom of their own, and walled members of Monera and Protista, such as bacteria and diatoms, stay in those kingdoms. Blue-green algae are prokaryotes, so today they are cyanobacteria in Monera and are not counted as algae at all. Five groups remain in Plantae: algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

How plants have been classified

Sorting the plants that remain has gone through three broad phases. The first schemes, now called artificial systems, sorted plants by whatever struck the eye: growth form (tree, shrub or herb), flower colour, how many leaves and what shape. Linnaeus built his famous system mainly on the androecium, counting and arranging the stamens. Such schemes are quick to use, but they mislead.

Imagine grouping plants by habit alone. A 'climbers' group would hold money plant, bitter gourd and black pepper, which come from three different families. Meanwhile one species that creeps along the ground on a windy hillside but grows upright in a sheltered garden could land in two groups. A few characters split close relatives and lump strangers together. Artificial systems also gave vegetative features the same weight as reproductive ones, although leaves and stems respond far more to the environment than flowers do.

Natural systems came next. They ask how closely plants are really related, their natural affinity, and judge it from as many characters as possible: outward form, and also evidence from inside the plant, from ultrastructure and anatomy to embryology and plant chemistry (phytochemistry). For flowering plants the best-known natural system is the one drawn up by George Bentham and Joseph Dalton Hooker.

The modern goal is a family tree. A phylogenetic system places organisms by descent, so a taxon should hold the descendants of one common ancestor. Where fossils show the branching, that is straightforward; where the fossil record is silent, taxonomists turn to other lines of evidence:

• Numerical taxonomy scores a plant on every character that can be observed, records each score as a number or code, and lets a computer compare plants across the whole table. No character outweighs another, and the table can run to hundreds of characters (Example 3.1).
• Cytotaxonomy uses evidence from cells: the number of chromosomes, their structure and how they behave.
• Chemotaxonomy compares the chemicals plants make, such as pigments, oils or alkaloids, to settle cases that structure alone leaves unclear.

3.1 Algae

Wherever light and moisture meet, algae (शैवाल) turn up: in ponds, rivers and the sea, and also as green films on wet stones, damp soil and soaked wood. They are the simplest members of Plantae. Each has chlorophyll, makes its own food and has a thallus, a body with no division into root, stem and leaf. Some team up with fungi to form lichens, and some even grow on animals: a green alga lives in the fur of tree sloths in Central and South American forests.11NCERT's text says 'sloth bear'. The documented case is the tree sloth, whose fur carries green algae; the sloth bear is an Indian bear.

Algal bodies span an astonishing range of form and size Fig. 3.2. Chlamydomonas is one swimming cell. Volvox is a colony, a hollow ball of cells. Ulothrix and Spirogyra are filaments, cells joined end to end. At the far extreme, marine kelps build massive bodies that form underwater forests.

Algae reproduce in three ways. In vegetative reproduction the thallus simply breaks up, and each fragment grows into a new thallus. In asexual reproduction they form spores; the commonest are zoospores, which carry flagella, swim for a while, then settle and grow into new plants. Sexual reproduction joins two gametes, and algae show every degree of difference between the two partners Fig. 3.3:

• Isogamy: the partners are the same size. In Ulothrix both swim with flagella; in Spirogyra neither has flagella.
• Anisogamy: both partners swim, but one is clearly bigger than the other, as in some species of Eudorina.
• Oogamy: the female gamete, the egg, is large and stays put, while a much smaller male gamete swims to it, as in Volvox and Fucus.

Botanists split algae into three main classes, chiefly by the pigments they carry and the food they store; the cell wall and the flagella help to confirm the choice. The classes are Chlorophyceae (green), Phaeophyceae (brown) and Rhodophyceae (red).

3.1.1 Chlorophyceae

Of the three classes, green algae are the most like land plants: their grass-green colour comes from the same pair of pigments, chlorophyll a and b, held in definite chloroplasts. Their bodies may be single cells, colonies or filaments. Under the microscope the chloroplast's shape is a handy clue to the species Fig. 3.4: a cup, a spiral ribbon, a net (reticulate), many small discs, or a flat plate.

Most chloroplasts contain one or more pyrenoids, storage bodies made of protein with starch around them; a few species keep oil droplets as well. The firm cell wall has two layers: pectose outside and cellulose inside.

Reproduction runs through the whole algal repertoire. Pieces break off and regrow; zoospores with flagella are released from zoosporangia; and, depending on the species, gametes fuse by isogamy, anisogamy or oogamy. Familiar genera include Chara, Spirogyra, Ulothrix, Volvox and Chlamydomonas.

3.1.2 Phaeophyceae

Brown algae include many of the large seaweeds of rocky coasts, and almost all are marine. Sizes run from the branched filaments of Ectocarpus to kelps tens of metres long.22NCERT gives kelps a height of up to 100 m, and exam questions follow that figure. Measured giant kelps are usually reported at up to about 45–50 m. A large brown alga is organised rather like a plant without being one. A holdfast grips the rock, a stalk called the stipe rises from it, and leaf-like fronds carry out photosynthesis (the last step of Fig. 3.2).

The brown colour is a matter of pigment balance. Alongside chlorophyll a and c, these algae carry carotenoids and xanthophylls, and one xanthophyll, fucoxanthin, decides the shade: the more of it, the browner the alga, from olive green to deep brown. Food is stored as complex carbohydrates, laminarin or mannitol. Each cell has a cellulose wall, usually with a jelly-like coat of algin outside, around plastids, a large central vacuole and a nucleus.

Besides fragmenting, brown algae release pear-shaped zoospores, each with two flagella of unequal length attached at the side of the cell rather than at its tip Fig. 3.5. The gametes look much the same: pear-shaped (pyriform), with two lateral flagella. Different species show isogamy, anisogamy or oogamy; oogamous species fuse their gametes inside the oogonium, the rest in open water. Fucus, Sargassum, Laminaria, Dictyota and Ectocarpus are common examples.

3.1.3 Rhodophyceae

Red algae owe their colour to a red pigment, r-phycoerythrin. Most are marine and they are commonest in warm seas. Many grow in bright water near the surface, yet some thrive at depths where very little light is left, deeper than any other seaweed Fig. 3.6. Phycoerythrin explains this. It absorbs the blue-green light that still reaches deep water and hands the energy on for photosynthesis.

Not one cell in a red alga's life swims. Its asexual spores and its gametes lack flagella, sexual reproduction is always oogamous, and fertilisation sets off an elaborate sequence of development. They also multiply by fragmentation. Most have multicellular thalli, some quite intricate, and they store floridean starch, a reserve built much like amylopectin and glycogen. Common examples are Gelidium, Gracilaria, Porphyra and Polysiphonia.

Table 3.3 puts the three classes side by side. Read down a column to describe one class, or along a row to compare all three on one feature.

3.2 Bryophytes

A few weeks into the monsoon, shaded walls, tree trunks and rocks in hill towns such as Darjeeling or Munnar turn velvety green. Most of that velvet is made of bryophytes: mosses and liverworts. They need damp, humid, shady places, and they are often called the amphibians of the plant kingdom: they live on land, but their sexual reproduction depends on water.

Look closely and a bryophyte seems to have roots, a stem and leaves. It does not, at least not true ones, because it has no vascular tissue; its root-like, stem-like and leaf-like parts only resemble those organs. Still, the body is more differentiated than an alga's. It is thallus-like, lying flat (prostrate) or standing erect, and rhizoids of one or many cells anchor it.

The green plant you see, a leafy moss shoot or a flat liverwort, is haploid ($n$). It makes the sex cells, so it is called the gametophyte. Its sex organs are not single cells, as in many algae, but multicellular structures Fig. 3.7:

• Antheridium (male): releases many antherozoids, each swimming with two flagella.
• Archegonium (female): a flask with a long neck and a swollen base that holds a single egg.

Rain sets the next stage going. Antherozoids swim through the film of water on the plant until one enters the neck of an archegonium and fuses with the egg, giving a diploid ($2n$) zygote. What happens next is new. Instead of dividing by meiosis at once, the zygote grows by mitosis into a multicellular sporophyte, a second generation that never leaves its parent: it stays fixed to the green gametophyte and takes its nourishment from it. Only later do certain sporophyte cells undergo meiosis, giving haploid spores that grow into new gametophytes.

Their direct economic value is small, with one notable exception: peat. Bogs of the moss Sphagnum build up peat, long cut and burnt as fuel, and dried Sphagnum holds so much water that it has been used to pack living plants for long journeys. Some mosses are also grazed by herbivorous mammals and eaten by birds.

Their ecological role is bigger still. Think of a bare boulder exposed by a new road cutting. Lichens and mosses are usually the first to settle on it. Over the years their growth and decay crumble the rock surface and trap dust and organic matter, building a thin soil in which grasses and later shrubs can root. That makes them key players in plant succession. On slopes, a thick carpet of moss takes the force of falling raindrops, so less soil is washed away. Bryophytes fall into two groups: liverworts and mosses.

3.2.1 Liverworts

Look for liverworts where it never quite dries out: beside streams, on marshy or damp soil, on tree bark and in the shade of dense forest. In Marchantia the body is a flat, forked thallus. It is dorsiventral, meaning its upper and lower surfaces differ, and it lies pressed close to the ground Fig. 3.8. Leafy liverworts look quite different: a stem-like axis carries tiny leaf-like scales in two rows.

A liverwort can spread without sex at all. The simplest way is for an old thallus to break up into pieces that keep growing. The neater way uses gemmae (singular gemma): small, green, multicellular buds that form inside gemma cups on the upper surface. Raindrops splash them out, and each gemma that lands on damp ground grows into a new plant, genetically identical to its parent.

Depending on the species, one thallus may carry both antheridia and archegonia, or the sexes may grow on separate thalli, as in Marchantia, where they sit on stalked, umbrella-like branches. Either way, the sporophyte that follows fertilisation is built from three parts: a foot anchored in the gametophyte, a short stalk (the seta) and a capsule in which meiosis makes the spores. Released spores germinate into new, free-living gametophytes.

3.2.2 Mosses

In mosses, too, the gametophyte dominates the life cycle. It grows up in two stages, and both are haploid:

• Protonema, the juvenile stage: the first thing a germinating spore produces. Picture a fine green mat of threads creeping over the soil; they branch and are usually filamentous.
• Leafy stage, the adult: buds form along the sides of the secondary protonema, and each bud grows upwards into a slim shoot. Its leaves spiral around the axis, and multicellular, branched rhizoids anchor it in the soil. Only this stage makes sex organs.

A moss sporophyte has the same three parts as a liverwort's, foot, seta and capsule, but it is more elaborate and especially good at scattering spores. In many mosses a ring of teeth round the mouth of the capsule bends open in dry air, when spores travel far, and closes again in damp air. The spores inside come from meiosis. The sporophyte itself grows from a zygote formed at the tip of a leafy shoot, where the antheridia and archegonia develop. Mosses can also multiply without sex, by fragmentation and by budding of the secondary protonema. Funaria, Polytrichum and Sphagnum are common mosses.

3.3 Pteridophytes

Ferns crowd the shady road cuttings of the Western Ghats and the north-east, and horsetails grow along streams; both are pteridophytes. People grow them as ornamentals, use some as medicines and plant others as soil-binders on slopes. Most need cool, damp, shady places, though a few manage on sandy soil.

Pteridophytes made an evolutionary leap: they are the first land plants with vascular tissues, xylem to carry water and phloem to carry food. You will study these tissues in Chapter 6. Vascular tissue changes the whole body plan, because a plant that can lift water and move food over long distances can grow large and build true organs.

Pteridophytes turn the bryophyte arrangement upside down. The fern you see is the sporophyte ($2n$): it has true roots, stems and leaves, each with well-developed vascular tissue, while the gametophyte has shrunk to a small, short-lived stage. Leaf size varies a lot: Selaginella carries tiny leaves called microphylls, while a fern frond is a big leaf, a macrophyll.

Spores are made in sporangia, inside which spore mother cells divide by meiosis. The sporangia are not scattered at random: each sits on, or in the angle of, a leaf-like sporophyll. In Selaginella and Equisetum the sporophylls are crowded into compact cones, the strobili; in ferns they look like ordinary leaves, with sori underneath.

Each spore grows into a gametophyte you could easily overlook: the prothallus, a flat green plate under a centimetre across, often heart-shaped in ferns Fig. 3.9. It is multicellular, makes its own food and lives on its own, but it dries out easily, so it survives only where it is cool, damp and shaded. On its underside it carries antheridia and archegonia. The antherozoids reach an archegonium only by swimming through a film of water; one of them fertilises the egg, and the zygote grows into a new sporophyte with well-differentiated organs, the dominant phase. With a fragile gametophyte and swimming gametes, living pteridophytes stay restricted to rather narrow geographical regions.

Selaginella and Salvinia deserve a closer look, because they break the usual pattern. A typical pteridophyte is homosporous: all its spores are alike. These two genera show heterospory instead, making large megaspores, which grow into female gametophytes, and small microspores, which grow into male ones Fig. 3.10.

Two further habits make them special. The female gametophyte is not simply shed to fend for itself but stays on the parent sporophyte for some time, and the zygote grows into a young embryo inside that female gametophyte. This is a forerunner of the seed habit, one of the great steps in plant evolution. A seed, which you will meet in gymnosperms, keeps the female gametophyte and the young embryo on the parent until they are well protected.

Pteridophytes are traditionally placed in four classes (Table 3.5).

3.4 Gymnosperms

The word gymnosperm (अनावृतबीजी) joins two Greek words, gymnos (naked) and sperma (seed). No ovary wall covers their ovules, which lie open on the sporophylls from the start and stay open after fertilisation, so the seeds formed from them are uncovered, or naked. Deodar forests in Himachal, chir pine on the hills of Uttarakhand and the palm-like Cycas planted in gardens across India are all gymnosperms.

Gymnosperms range from shrubs to some of the tallest trees on Earth: the tallest known living tree is a coast redwood, Sequoia, about 116 m high. Most have tap roots. Picture a Cycas and a pine side by side to see how much they vary:

• Cycas: an unbranched trunk crowned with large pinnate (compound) leaves that last a few years, and small, specialised coralloid roots near the surface that house cyanobacteria fixing nitrogen ($\mathrm{N}{\phantom{X}}_2$).
• Pinus: a branched tree, like Cedrus, with simple needle leaves, and roots that live in partnership with fungi as mycorrhiza.

Gymnosperm leaves cope with extremes of temperature, humidity and wind. A pine needle is a leaf built for hardship. Its narrow shape keeps the surface area small, a thick cuticle seals it, and its stomata are sunken in pits, out of the drying wind.

Reproduction centres on cones (strobili). A cone is an axis with sporophylls spiralling round it, loosely or tightly packed, and the sporophylls carry sporangia. Like Selaginella, every gymnosperm is heterosporous, making two kinds of haploid spore, microspores and megaspores, so there are two kinds of cone:

• Male cones (microsporangiate strobili) carry microsporophylls with microsporangia. Inside the microsporangium a microspore develops into a male gametophyte so reduced that it has only a few cells: the pollen grain.
• Female cones (macrosporangiate strobili) carry megasporophylls that bear ovules.

In Pinus both kinds of cone grow on the same tree. Cycas is different: male cones form on some plants, and megasporophylls, loosely clustered rather than built into a compact cone, form on others.

Now look inside an ovule Fig. 3.11. Its core is the nucellus, the megasporangium, wrapped in protective layers called integuments. One nucellus cell becomes the megaspore mother cell, which divides by meiosis into four megaspores. Only one of the four survives. Still inside the megasporangium, it grows into a multicellular female gametophyte bearing two or more archegonia.

Notice what has happened to the gametophytes. In a moss the gametophyte was the whole plant, and in a fern a free-living prothallus; in a gymnosperm both gametophytes stay inside sporangia on the sporophyte and never live on their own. Wind does the carrying. Clouds of pollen leave the microsporangia, and the few grains that settle at the micropyle, the opening of an ovule, send out a pollen tube. The tube grows towards the archegonia and empties its male gametes near their necks, so no water is needed for fertilisation.33In pine, pollination happens while the ovule is still young; the female gametophyte matures, and fertilisation follows roughly a year later. After fertilisation the zygote grows into an embryo, and the ovule matures into a seed. True to the group's name, no fruit covers that seed.

3.5 Angiosperms

Angiosperms (आवृतबीजी) are the flowering plants, and the flower is their defining invention: both the pollen grains and the ovules are made inside it. The ovules sit sealed in an ovary, and after fertilisation the seeds stay sealed in, now inside a fruit Fig. 3.12. That difference, seeds inside a fruit rather than seeds lying on an open scale, is why gymnosperms and angiosperms are classified separately although both bear seeds.

No other plant group comes close in numbers: angiosperms have spread into almost every habitat on land and into many in fresh and salt water. Size is no limit either, from Wolffia, a rootless floating plant less than about a millimetre long and the smallest flowering plant, to Eucalyptus trees about 100 m tall. Nearly all our food crops, most fodder, much fuel wood and medicine, timber and fibres such as cotton and jute come from them.

Flowering plants fall into two classes named after the number of seed leaves (cotyledons) in the embryo: dicotyledons have two and monocotyledons have one. The comparison below gives quick field clues; you will study these features properly in Chapter 5.

Looking across the plant kingdom

Line up the four land-plant groups and two trends stand out Fig. 3.13. The gametophyte shrinks, from the whole leafy plant in a moss to a few cells hidden inside a flower. The sporophyte grows, from a capsule riding on the gametophyte to a forest tree. Alongside, dependence on water for fertilisation ends once pollen and pollen tubes appear, and the seed arrives and is later wrapped in a fruit.

The sorter below drills the two skills that exam questions test most often: placing a genus in its group, and naming the ploidy of a structure.